Small portable electronic devices that contain a speaker cannot generate significant open-air acoustic output at frequencies below approximately 250 Hz (the low-frequency cutoff point, 262 Hz corresponds to “middle C”). This limitation negatively affects the sound of these devices (giving it a thin, unnatural and somewhat unpleasant character) and also prevents the transmission of important musical content below the cutoff frequency. At present, the discriminating user must connect his device to large external speakers or resort to headphones to enjoy the full audio experience.
Extending the open-air (ambient) frequency response below 250 Hz requires a larger diaphragm to move more air and an increasing enclosure volume. The moving diaphragm generates pressure waves perceived by the ear. In general, as the enclosure volume increases, the frequency range of the system (over which the diaphragm can effectively resonate and produce significant acoustic energy) extends to lower frequencies. In a manner proportional to the volume, the size and weight of the system also increase and the speaker's portability is compromised.
The Samsung ESP-210 product includes a small stereo speaker pair on a baffle that folds along an axis perpendicular to the line joining the centers of the left and right speakers. The folding aspect of the product has no bearing on the frequency response of the system. Power is 3AAA or u-USB. There are other brands of product using this configuration. An important point is that the concept of “folding” when applied to portable speakers generally refers to collapsing the relative location of the drivers with respect to each other, rather than reducing the volume of the acoustic enclosure itself.
The XMI X-Mini 2 product includes a roughly 3″ spherical speaker system that pops open along a plane passing through its center. The 40 mm (1.6″) driver points upward from the top of the upper hemisphere. The two hemispheres are connected by a pleated tube which maintains the enclosure volume, which appears to increase slightly when the device is open. It has a rechargeable battery and can be connected to a USB power source.
The XMI X-Mini Max 2 product includes a cone-shaped capsule version of the product just above to provide a “bass expansion system” that “creates an extendable vacuum, which is capable of producing bass over ten times what the physical size of these speakers would normally allow.” The claimed frequency response is 200-18 kHz. The power is 2.5 W for each speaker. The impedance is 4 ohms. There is a built-in rechargeable battery with a 2.5 hr charge time and 12 hr playback time.
The Satechi SX2 product is roughly cylindrical and pops open similar to the XMI products above.
The Bose Soundlink Wireless Mobile Speaker includes a larger fixed-size battery-powered speaker/amplifier marketed as portable. The size of this product is 5″ H×10″ W×2″ D (100 ci) and the weight is 3 lbs (price is $300). There is no frequency response or amplifier power information in the specification or manual. The speaker contains 4 small (˜1.8″) long-throw drivers (2 one each side), with 2 opposed passive radiators (˜3″×5″) in the center pointing front and rear. This product also contains 2 circuit boards, one for button interface on the top and the other for the wireless receiver, processor and amplifier. There is a tubular-shaped battery compartment in the bottom. A full charge is said to power the unit at full volume for 3 hours, and the recommended charging time is also 3 hours. The charger rating is 17V 1 A, so the Li-Ion battery may be composed of 4× CR123 cells. There is also a usb “service connector” which is only for software updates, and cannot be used to charge or play music.
The Wowee One Power Bass Portable Speaker is a small (60×120×26 mm) battery powered speaker system containing a mid-high frequency driver and a “gel audio” (SFX Technologies Ltd) low-frequency driver which (in combination with an external flat surface) is claimed to deliver frequency response down to 40 Hz. Adhesive pads are used to affix the unit to the surface and prevent vibration. The device contains an internal Li-Ion battery and 2 W class-D amplifier. Inputs include a ⅛″ phone jack and mini-usb IEEE 1394 power jack. 2nd generation products are enhanced with a slimmer profile (16 mm thick) and/or Bluetooth wireless capability.